1. Field of the Invention
This invention relates to a material of a photo cathode used in an electron image projection apparatus, in which electrons emitted from the patterned photo cathode are accelerated and focused on a surface of semiconductor wafer to be patterned, by a high voltage and magnetic field applied perpendicular to the surface of the mask as well as the wafer. More particularly, this invention relates to an improvement of the material in that it is more uniform, as well as less susceptive to the atmosphere, in characteristics.
2. Description of the Related Art
Photo lithography has been an important technique in the production of integrated circuits (IC). However, among the photo lithography techniques, in order to satisfy recent demand for a sub-micron resolution, an electron image projection technique was proposed by T. W. O'Keefe, et al in "An Electron Imaging System for the Fabrication of Integrated Circuits", Solid State Electron., vol.12, p.841, 1969. This subject and has been intensively studied and developed by many people, such as reported by I. Mori et al. in "An Electron Beam-Image-Projection System for Submicron", Toshiba Review, vol.41, No.5, 1986.
The main structure of the electron image projection apparatus in vacuum (10.sup.-9 to 10.sup.-6 Torr) is schematically illustrated in FIG. 1. A mask substrate 11 which has been patterned in advance with a photo cathode material as will be described below is held by a mask holder 22. A semiconductor wafer 31 on which the pattern on the mask substrate 11 is to be copied is placed facing toward and parallel to the mask substrate 11 approximately 30 mm distant, on a wafer holder 30. An ultra violet light 38 of a proper wave length is irradiated onto the surface of the mask substrate 11, reflected by reflectors 36, or onto the back surface of the mask substrate made of a transparent material by the use of an apparatus which is not shown in figure. Then the patterned photo cathode is photo-electrically excited and emits photo electrons, which are accelerated by the applied voltage of approximately 80 KV between the mask substrate 11 and the wafer 31. This voltage 80 KV is applied by supplying -80 KV onto the mask substrate 11 and 0 V onto the wafer 31. On the other hand, a magnetic field is applied perpendicular to the surface of the mask substrate 11 as well as the wafer 31. The emitted and accelerated electrons 39 are thus accurately focused on the surface of the wafer 31 by the electric and magnetic field applied thereto. This magnetic field is applied from magnetic poles 33, 34 and a compensation coil 26 for providing a uniform magnetic field. An E-beam resist material, such as PMMA (poly methyl metacrylate), coated on the semiconductor wafer 31 is exposed by thus accelerated and focused electrons precisely the same as the pattern of the photo cathode on the mask substrate 11. This is the principle of the electron image projection.
As for the photo cathode material used for the electron image projection, there are two major types. The first one is CsI (cesium iodide) and the second one is a semiconductor material, such as GaAs (gallium arsenide), having Cs (cesium), etc. thereon, as previously applied by Yasuda et al. for U.S. patent application No. 799,321 now abandoned in favor of continuing U.S. patent application No. 144,275, now U.S. Pat. No. 4,789,786.
Structure of the first type of photo cathode using back-illuminating light is schematically illustrated in FIG. 2. Upon a substrate 15 which is made of quartz or sapphire and is therefore transparent to the illuminating ultra violet light, a Cr (chrome) layer is patterned as denoted by 12 by a conventional photo mask making technique, such as sputtering, an electron beam exposure and etching. Then, CsI is deposited upon all the surface of the patterned substrate 15, as denoted by 10 and 10'. An ultra violet light 4 is irradiated onto the back surface of the substrate 15, through which the light photo-electrically excites the CsI 10 coated directly on the substrate, indicated as a photo cathode 14 in FIG. 2. Then, the light-excited CsI 10 emits photo electrons, which are pulled out of the surface by the applied electric field. The Cr pattern 12 prevents light from arriving at the CsI layer 10' thereon. Thus, the photo cathode 14 is made patterned.
Structure of the second type of photo cathode using surface-irradiating light is schematically illustrated in FIG. 3. Upon a substrate 11 made of a photoelectric material, such as GaAs, which is not transparent to the illuminating light, a tungsten layer for masking a non cathode portion is patterned as indicated by the numeral 12' by a conventional lithography technique, such as sputtering, an electron beam exposure and etching. Then, several atomic layers of Cs 13 and 13' are coated all over the surface of the tungsten-patterned substrate 11. An ultra violet light 4' is irradiated onto the surface of the patterned substrate 11 having the Cs layer 13 directly thereon. Then a photo cathode portion 14, where the GaAs substrate 11 absorbs the irradiated light 4' through the Cs layer 13 and produces photo electrons therein, emits electrons, which are pulled out of the surface by the applied electric field. The tungsten pattern 12' having Cs atoms 13' thereon emits fewer photo electrons than the GaAs having Cs thereon. Thus, the photo cathode 14 is formed patterned.
Problems of the CsI are as follows. (1) The work function for emitting a photo electron is as high as approximately 6 eV, which requires a low-pressure mercury lamp as a light source emitting a light of approximately 1849 .ANG. wave length for irradiating the photo cathode. However, there is no suitable focusing means available for the light of this wave length. (2) Life of CsI is not long enough for an efficient fabrication process of the ICs. It is because the CsI is not only very deliquescent but also reduces itself even in vacuum. This fact also makes it impossible to take out the CsI-coated mask into open air for example, inspection of the uniformity of the CsI coating.
Problems of the GaAs plus Cs is as follows. After a masking pattern is formed with tungsten etc. by a widely known method, such as sputtering, lithography and etching process, on the surface of the GaAs substrate, the surface of the GaAs substrate must be cleaned up by being heated at 630.degree. C. or higher in a high vacuum, so that a contaminating material on the substrate is evaporated. Accordingly, the atoms of the substrate material can contact Cs atoms coated thereon for reducing the work function of the surface of the photo cathode. However, this heat cleaning is not always reliable to provide a satisfactorily clean surface. This causes less uniform, i.e. less stable, emissivity of the photo electrons therefrom. Further more, this heating process makes the masking metal diffuse laterally into the cathode portion 14 which is not masked thereby. Thus, the border line between the masked area and not-masked portion, i.e. 14, is made less sharp, namely blurring, by this diffusion of the masking metal. At this blurring edge portion, the emissivity of the photo electrons is deteriorated, resulting in a deterioration of the sharpness, i.e. resolution, of the image projection.